Polyurethane systems include, for example, polyurethane coatings, polyurethane adhesives, polyurethane sealants, polyurethane elastomers or polyurethane foams.
Owing to their excellent mechanical and physical properties, polyurethane foams are used in a wide variety of fields. A particularly important market for various types of PUR foams, e.g., conventional flexible foams based on ether polyols and ester polyols, high-resilience foams (frequently referred to as HR foams), rigid foams, integral foams and microcellular foams, and also foams whose properties lie between these classifications, e.g., semirigid systems, is the automobile industry and the furniture industry. For example, rigid foams are used as roof lining, ester foams are used for interior cladding of doors and for stamped-out sun visors, while high-resilience foams and flexible foams are used for seat systems and mattresses.
Catalysts suitable for one-component moisture-reactive polyurethane compositions typically contain tin compounds such as tin carboxylates, in particular tin octoate (corresponds to tin 2-ethylhexanoate), frequently combined with tertiary amines.
The use of tin octoate in the production of flexible PUR foams based on polyetherols is described, for example, in Steve Lee, Huntsman Polyurethanes, The Polyurethanes Book, Verlag Wiley, pp. 140, 143-144, and Ron Herrington, Flexible Polyurethane Foams, Dow Chemical, pp. 2.30. The tin octoate serves as a catalyst for the reaction of isocyanates with polyols (also referred to as a gelling catalyst) via a complex transition state. During production of the foam, the tin octoate hydrolyses and liberates both the salt of 2-ethylhexanoic acid and also the acid itself. The decomposition is desirable because the backreaction of urethane formation to reform the starting materials is suppressed, but it should not, where possible, lead to liberation of possibly toxicologically problematical substances. The patent literature, too, contains numerous patent applications which describe the use of said tin octoate, e.g., see BE 779607, GB 1432281, GB 1422056, GB 1382538, GB 1012653, GB 982280. In these documents, catalyst systems comprising tin octoate are preferably used.
However, such tin catalysts have recently been coming under increasing pressure from users because of toxicological concerns regarding the starting materials, in particular the ligands, used for producing them. There is therefore an increasing need for less toxicologically problematical alternatives.
To meet the requirements, which have become significantly more demanding in recent years, in which the automobile and furniture industries and their foam suppliers have to meet in respect of emission and toxicity specifications, catalyst systems that contain less toxic ligands which can be polymerized into the foam have been developed. Such systems are described, for example, in EP 1013704. The disadvantage of these systems is the higher amounts needed and the associated costs because of the lower tin content and the strong shielding of the active tin by the ligands. The systems have to date been one of the few alternatives to the widespread tin octoate catalyst system (tin(II) salt of 2-ethylhexanoic acid) or organotin compounds such as dibutyltin dilaurate. The latter systems are to be viewed critically because of the toxicity of the substances emitted. 2-ethylhexanoic acid, for example, which is liberated during and after foaming gives cause for concern because of possible damage to an unborn child (development damage) in human beings (R 63).
Bismuth catalysts represent a further alternative to conventional tin catalysts. Bismuth catalysts which are known for polyurethane compositions are, for example, bismuth carboxylates, e.g., bismuth octoate (bismuth salt of 2-ethylhexanoic acid), as mentioned in WO 98/36007. However, the catalytic activity of bismuth compounds in respect of the isocyanate-water reaction is significantly lower than that of tin catalysts and the emission of 2-ethylhexanoic acid is a problem in these catalyst systems as well.
A further disadvantage of the stated catalyst systems is their very narrow processing latitude. It has been observed that, in corresponding catalyst systems, a catalyst system which is varied slightly to higher use amounts leads to very closed-celled foams or severe shrinkage.